Mechanical and chemical-mechanical planarizing processes (collectively “CMP”) remove material from the surface of semiconductor wafers, field emission displays or other microelectronic substrates in the production of microelectronic devices and other products. FIG. 1 schematically illustrates a CMP machine 10 with a platen 20, a carrier assembly 30, and a planarizing pad 40. The CMP machine 10 may also have an under-pad 25 attached to an upper surface 22 of the platen 20 and the lower surface of the planarizing pad 40. A drive assembly 26 rotates the platen 20 (indicated by arrow F), or it reciprocates the platen 20 back and forth (indicated by arrow G). Since the planarizing pad 40 is attached to the under-pad 25, the planarizing pad 40 moves with the platen 20 during planarization.
The carrier assembly 30 has a head 32 to which a substrate 12 may be attached, or the substrate 12 may be attached to a resilient pad 34 in the head 32. The head 32 may be a free-floating wafer carrier, or an actuator assembly 36 may be coupled to the head 32 to impart axial and/or rotational motion to the substrate 12 (indicated by arrows H and I, respectively).
The planarizing pad 40 and a planarizing solution 44 on the pad 40 collectively define a planarizing medium that mechanically and/or chemically removes material from the surface of the substrate 12. The planarizing pad 40 can be a soft pad or a hard pad. The planarizing pad 40 can also be a fixed-abrasive planarizing pad in which abrasive particles are fixedly bonded to a suspension material. In fixed-abrasive applications, the planarizing solution 44 is typically a non-abrasive “clean solution” without abrasive particles. In other applications, the planarizing pad 40 can be a non-abrasive pad composed of a polymeric material (e.g., polyurethane), resin, felt or other suitable materials. The planarizing solutions 44 used with the non-abrasive planarizing pads are typically abrasive slurries with abrasive particles suspended in a liquid.
To planarize the substrate 12 with the CMP machine 10, the carrier assembly 30 presses the substrate 12 face-downward against the polishing medium. More specifically, the carrier assembly 30 generally presses the substrate 12 against the planarizing liquid 44 on a planarizing surface 42 of the planarizing pad 40, and the platen 20 and/or the carrier assembly 30 move to rub the substrate 12 against the planarizing surface 42. As the substrate 12 rubs against the planarizing surface 42, material is removed from the face of the substrate 12.
CMP processes should consistently and accurately produce a uniformly planar surface on the substrate to enable precise fabrication of circuits and photo-patterns. During the construction of transistors, contacts, interconnects and other features, many substrates develop large “step heights” that create highly topographic surfaces. Such highly topographical surfaces can impair the accuracy of subsequent photolithographic procedures and other processes that are necessary for forming sub-micron features. For example, it is difficult to accurately focus photo patterns to within tolerances approaching 0.1 micron on topographic surfaces because sub-micron photolithographic equipment generally has a very limited depth of field. Thus, CMP processes are often used to transform a topographical surface into a highly uniform, planar surface at various stages of manufacturing microelectronic devices on a substrate.
In the highly competitive semiconductor industry, it is also desirable to maximize the throughput of CMP processing by producing a planar surface on a substrate as quickly as possible. The throughput of CMP processing is a function, at least in part, of the polishing rate of the substrate assembly and the ability to accurately stop CMP processing at a desired endpoint. Therefore, it is generally desirable for CMP processes to provide a controlled polishing rate (a) across the face of a substrate to enhance the planarity of the finished substrate surface, and (b) during a planarizing cycle to enhance the accuracy of determining the endpoint of a planarizing cycle.
One concern of CMP processing is that it is difficult to control the polishing rate. The polishing rate typically varies across the surface of the workpiece or during a planarizing cycle because (a) topographical areas with high densities of small features may polish faster than flat peripheral areas, (b) the distribution of abrasive particles in the slurry varies across the face of the workpiece, (c) velocity and thermal gradients vary across the surface of the workpiece, (d) the condition of the surface of the planarizing pad varies, (e) the topography of the workpiece changes, and (f) several other factors. The variance in the polishing rate may not be uniform across the workpiece, and thus it may cause different areas on the workpiece to reach the endpoint at different times. This produces over-polishing in areas with high polishing rates, and under-polishing in other areas with lower polishing rates.
The variance in the polishing rate can be particularly difficult to control when slurries with very small abrasive particles are used on wafers with a high density of small features. It is becoming increasingly important to use very small abrasive particles in CMP slurries because the feature sizes of the microelectronic components are decreasing to produce high performance/capacity products, and the small particle sizes enable mechanical removal of material from workpieces without damaging or otherwise impairing the small components. The slurries with small particle sizes, however, may produce different results as the surface of the planarizing pad changes throughout a run of workpieces, or even during a single planarizing cycle of one workpiece. This can produce inconsistent results that reduce the reliability of CMP processing. Therefore, there is a strong need to provide a planarizing process that can accurately endpoint a planarizing cycle without significantly increasing the time to planarize each workpiece.